1. Field of the Invention
This invention relates to a plastic screw anchor used for receiving a self-tapping screw to fasten and secure a structural element (e.g. a plate or small mechanical part) to a support member by the self-tapping screw.
2. Description of Background Art
It is a recent tendency to make many kinds of mechanical parts from synthetic resin for the sake of cost reduction, weight reduction and improvement of corrosion resistance and appearance.
The screw anchor is one of the mechanical parts which has been developed in line with this tendency as seen from those disclosed, for example, in the U.S. Pat. Nos. 2,788,047, 2,836,214 and 2,956,605.
However, the conventional plastic screw anchor has several disadvantages as follows.
Firstly, since the legs of the conventional plastic screw anchor are formed thick and stiff, they are therefore not easily deformable in accordance with the dimensions of an aperture formed in a supporting plate when the legs are inserted into the aperture of the supporting plate. Because of this, careful attention must be given to accurately forming the aperture so that the legs of the screw anchor smoothly fit into the aperture. This is very troublesome and reduces working efficiency of the screw anchor.
Furthermore, a strong pressing force is required for inserting the legs of the screw anchor into the aperture of the supporting plate when the tolerance between the aperture and the legs is small.
Secondly, the conventional screw anchor is liable to fall out from the aperture of the supporting plate after having been once inserted thereinto when the tolerance between the aperture and the legs is large.
Thirdly, the gripping engagement between the conventional screw anchor and the supporting plate is unreliable, since the legs of the screw anchor are stiff so that the edges of the aperture cannot easily bite into the legs when a self-tapping screw has been screwed into the screw anchor.